作者单位:1. Department of Mathematics, Stevens Institute of Technology, Room 302, Peirce Building, Castle Point on Hudson, Hoboken, NJ 07030, USA2. Department of Management Science and Information Systems, Rutgers University, 251 J.H. Levin Building, 94 Rockafeller Road, Piscataway, NJ 08854, USA3. Institute of Mathematics, Department of Differential Equation, Budapest University of Technology and Economics, Egry József utca 1, H ép. IV. em., 1111 Budapest, Hungary